The invention is directed to a micromechanical component part having a static micromechanical structure and a movable micromechanical structure; and to a method for the manufacture thereof.
There currently exists a great interest in processes that are compatible with the fabrication of integrated circuits, particularly of silicon, in the manufacture of micromechanical component parts for employment as actuators or sensors. A compatibility of manufacturing processes allows integration of micromechanics and drive circuits in microsystems. This is also important when existing semiconductor manufacturing systems are to be simultaneously utilized for the manufacture of micromechanical structures. Micromechanical component parts whose function is based on electrostatic attractive forces, such as between capacitor surfaces with variable spacing, are particularly suited for such microsystems, since a suitable combination of conductive and non-conductive layers, that are also utilized in integrated circuits, is thereby fundamentally adequate.
Such micromechanical component parts are composed of static and movable micromechanical structures; a typical applied example thereof is a motor (with rotational axle and stator as static and, respectively, with rotor as movable micromechanical structure) or a gearing (with rotational axle or, respectively, rotor).
The following problems must be solved in the manufacture of, for example, a motor:
Since the electrostatic forces decrease with increasing spacing, optimally small spacings between rotor and stator are desirable. This and the general mechanical tolerance demands require slight adjustment tolerances of the appertaining photo levels. Further, high-resolution lithography is important. Deriving therefrom as a further demand is an optimally far-reaching planarity of the surfaces during the manufacturing process. This is also of significance for the following process steps. Since the electrostatic forces and the torque of a micromotor increase with the area, the manufacturing process should allow an optimally great thickness of rotor and stator in the direction of the rotational axle (in a gearing without electrical connection, too, a large layer thickness is desirable in order to assure mechanical stability). A low specific resistance of the conductive material is also important for the low-loss function of the motor. In general, a process should be optimally simple and require few photo levels.
Among others, the following manufacturing processes are known:
a) Polysilicon Center-Pin Process (M. Mehrengany et al., J. Micromech. Microeng., Vol. 1, 73, 1991).
In the manufacture of the micromechanical structures, this process requires additional polysilicon depositions after the metallization complex following the manufacture of the integrated circuit. If the center-pin process is to be implemented before the metallization of the integrated circuit, the problem of etching the movable structures free arises and the problem of protecting the insulation of the metallization simultaneously arises. Four photolevels are required. The high topography steps over which the third and fourth mask level must be produced as well as the employment of doped polysilicon that comprises a specific resistance of approximately 2 m.OMEGA. cm and a practically realizable layer thickness of less than 5 .mu.m, are disadvantageous.
b) Polysilicon Flange Process (M. Mehrengany, Y.--C. Tai, J. Micromech. Microeng., Vol. 1, 73, 1991). PA1 c) Polysilicon LOCOS Process (L. S. Tavrow et al, Sensors and Actuators A, 35 (1992) Page 33). PA1 d) Selective Tungsten Process (L. Y. Chen, N. C. MacDonald, TRANSDUCERS '91, International Conference on Solid-State Sensors and Actuators, San Francisco, 24-27 June 1991, IEEE Catalog No. 91CH2817-5, 739, 1991).
This process is similar to process a), whereby, moreover, the second mask level must be produced over a topography step.
This process provides that the movable micromechanical structures be produced on a planar LOCOS oxide layer, whereby the oxidation step can only ensure before the transistor manufacture because of the temperature stress. Given a complete implementation of the process before the manufacture of the integrated circuit, an unfavorable topology for the following steps derives, as does the problem of protecting the micromechanical elements during the manufacture of the integrated circuit. Given manufacture meshed with one another, the problem of etching the rotor free given simultaneous protection of the circuit's insulating oxides must be resolved. A further disadvantage is the employment of six mask levels, some of these having to be generated over high topography steps.
This process can be implemented following a circuit fabrication process, whereby the problem of contacting the motor and of protecting the metallization insulation is not resolved by the proposed method. Rotor and stator are in fact produced in the same mask level and the rotational axle is produced self-aligned relative thereto; the process, however, requires five mask levels, the third, fourth and fifth thereof having to be produced over high topography steps. High layer thicknesses of a highly conductive material can be produced by the deposition of tungsten in trenches.